Vehicle trajectory control system

ABSTRACT

A method for controlling an engine coupled to a transmission is described. The method is especially suited for a transmission having at least one gear in which an over-running clutch is present. The invention describes a method for adjusting an engine operating parameter to maintain transmission input speed at or below a synchronous transmission input speed when the transmission is in the gear with the over-running clutch. The synchronous transmission input speed is based on transmission state and transmission output speed. Alternatively, vehicle speed can be used in place of transmission output speed.

FIELD OF THE INVENTION

The present invention relates to a system and method to control apowertrain of a vehicle, and in particular powertrain control duringvehicle deceleration.

BACKGROUND OF THE INVENTION

A vehicle powertrain typically includes a power source such as aninternal combustion engine, coupled to a gearbox such as a transmissionwith various gear ratios. A powertrain controller is usually providedwhich controls the powertrain in part based on a driver's requests.

In one system, desired powertrain output is determined based ondepression of a pedal by a vehicle operator. In such a system, astrategy is needed for the situation when a driver releases the pedal,known to those skilled in the art as a tip-out. During a tip-out, thedriver is indicating a desire for reduced engine output.

The inventors herein have recognized a disadvantage if conventionalmethods of powertrain output control are used during tip-out situations.In particular, if the accelerator pedal is released and subsequentlyengaged, poor drivability may be experienced due to transmission gearslash. For example, when the engine transitions from exerting a positivetorque to exerting a negative torque (or being driven), the gears in thetransmission separate at the zero torque transition point. Then, afterpassing through the zero torque point, the gears again make contact totransfer torque. This series of events produces an impact, or clunk,resulting in poor drivability and customer dissatisfaction. In otherwords, the engine first exerts a positive torque through the torqueconverter onto the transmission input gears to drive the vehicle. Then,in response to the driver tip-out, the engine is driven by the torquefrom the transmission through the torque converter. The transitionbetween these two modes is the point where the engine is producingexactly zero engine brake torque. Then, at this transition point, thegears in the transmission separate because of inevitable transmissiongear lash. When the gears again make contact, they do so dynamically,resulting in an undesirable impact.

This disadvantage is exacerbated when the operator returns theaccelerator pedal to a depressed position, indicating a desire forincreased powertrain output. In this situation, the zero torquetransition point must again be traversed. However, in this situation,the engine is producing a larger amount of torque than duringdeceleration because the driver is requesting acceleration. Thus,another, more severe, impact is experienced due to the transmission lashduring the zero torque transition.

The inventors herein have recognized another disadvantage ifconventional methods of powertrain output control are used duringtip-out situations. In particular, different vehicle trajectories areobtained depending on road grade. Stated another way, if a desiredpowertrain output torque is requested, different vehicle decelerationrates are obtained depending on road grade. At steep down grades, thevehicle may accelerate even if negative powertrain torque is requesteddue to vehicle mass. If the conventional control strategy is changed tominimize this acceleration on down grades, then excessive decelerationis obtained on flat road grades. Thus, drive performance is sacrificedunder one driving condition or another.

SUMMARY OF THE INVENTION

The disadvantages of prior approaches are overcome by a method forcontrolling a powertrain coupled to a vehicle, the powertrain having aninternal combustion engine coupled to a transmission, the vehicleoperated by a driver. The method comprises determining a desired vehicletrajectory, adjusting an engine operating parameter to maintain positivepowertrain output when an actual vehicle trajectory is below saiddesired vehicle trajectory, and adjusting said engine operatingparameter to transition from positive powertrain output to negativepowertrain output when said actual vehicle trajectory is above saiddesired vehicle trajectory.

By maintaining positive powertrain output when an actual vehicletrajectory is below said desired vehicle trajectory, it is possible toprevent transmission separation and “clunk.” For example, during atip-out, if actual vehicle acceleration is less than desired vehicleacceleration, then it is not necessary to provide negative powertraintorque, and positive torque can be maintained to prevent “clunk.”

Alternatively, by transitioning from positive powertrain output tonegative powertrain output when actual vehicle trajectory is above saiddesired vehicle trajectory, it is possible to provide the desiredvehicle trajectory, i.e., drive feel may be optimized by allowing atransition through zero torque but maintaining the vehicle trajectory.Stated another way, a transition through zero torque is allowed tomaintain a desired vehicle trajectory.

An advantage of the invention is minimum transitioning from positive tonegative torque, thereby minimizing clunk.

Another advantage of the invention is that transitions from positive tonegative torque are performed when necessary to maintain a desiredvehicle trajectory. Thus, improved vehicle drivability is obtained.

Yet another advantage of the present invention is that when a transitionfrom positive to negative torque is necessary, a smooth transition isachieved.

It is also important to note that the desired vehicle trajectory caninclude various parameters such as, for example, a desired vehicleacceleration or deceleration, or a desired vehicle speed. Further,various engine operating parameters can be used to maintain positivepowertrain output such as, for example, engine airflow, engine air/fuelratio, engine torque, or engine ignition timing.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1-2 are block diagrams of an embodiment wherein the invention isused to advantage;

FIGS. 3-18 are high-level flow charts of various operations performed bya portion of the embodiment shown in FIG. 1;

FIG. 19 is a graph illustrating operation according to the presentinvention; and

FIGS. 20-23 are block diagrams of torque converters that can be usedaccording to the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, internal combustion engine 10, further describedherein with particular reference to FIG. 2, is shown coupled to torqueconverter 11 via crankshaft 13. Torque converter 11 is also coupled totransmission 15 via transmission input shaft 17. Torque converter 11 hasa bypass clutch (described in FIGS. 20-23), which can be engaged,disengaged, or partially engaged. When the clutch is either disengagedor partially engaged, the torque converter is said to be in an unlockedstate. Transmission 15 comprises an electronically controlledtransmission with a plurality of selectable discrete gear ratios.Transmission 15 also comprises various other gears such as, for example,a final drive ratio (not shown). Transmission 15 is also coupled to tire19 via axle 21. Tire 19 interfaces the vehicle (not shown) to the road23. In a preferred embodiment, transmission 15 has the following driverselectable options: park (P), reverse (R), neutral (N), driver (D), andlow (L). The driver selects these positions via a transmission lever. Inthis preferred embodiment, the lever is known as the PRNDL lever,corresponding to the different options. In particular, in park orneutral, transmission 15 does not transmit torque from the transmissioninput to the output. In drive, a transmission controller can controltransmission to select any available forward gear ratios. In reverse, asingle reverse gear is selected. In low, only a lower set of forwardgear ratios can be selected by the electronic controller. Those skilledin the art will recognize, in view of this disclosure, various othertypes of transmission levers with different sets of options that can beused with the present invention. For example, there can be low 1 and low2 options. Also, the transmission lever may be located on a steeringcolumn or between driver and passenger seats.

Internal combustion engine 10 comprises a plurality of cylinders, onecylinder of which is shown in FIG. 2. Electronic engine controller 12controls Engine 10. Engine 10 includes combustion chamber 30 andcylinder walls 32 with piston 36 positioned therein and connected tocrankshaft 13. Combustion chamber 30 communicates with intake manifold44 and exhaust manifold 48 via respective intake valve 52 and exhaustvalve 54. Exhaust gas oxygen sensor 16 is coupled to exhaust manifold 48of engine 10 upstream of catalytic converter 20.

Intake manifold 44 communicates with throttle body 64 via throttle plate66. Throttle plate 66 is controlled by electric motor 67, which receivesa signal from ETC driver 69. ETC driver 69 receives control signal (DC)from controller 12. Intake manifold 44 is also shown having fuelinjector 68 coupled thereto for delivering fuel in proportion to thepulse width of signal (fpw) from controller 12. Fuel is delivered tofuel injector 68 by a conventional fuel system (not shown) including afuel tank, fuel pump, and fuel rail (not shown).

Engine 10 further includes conventional distributorless ignition system88 to provide ignition spark to combustion chamber 30 via spark plug 92in response to controller 12. In the embodiment described herein,controller 12 is a conventional microcomputer including: microprocessorunit 102, input/output ports 104, electronic memory chip 106, which isan electronically programmable memory in this particular example, randomaccess memory 108, and a conventional data bus.

Controller 12 receives various signals from sensors coupled to engine10, in addition to those signals previously discussed, including:measurements of inducted mass air flow (MAF) from mass air flow sensor110 coupled to throttle body 64; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling jacket 114; a measurement ofthrottle position (TP) from throttle position sensor 117 coupled tothrottle plate 66; a measurement of turbine speed (Wt) from turbinespeed sensor 119, where turbine speed measures the speed of shaft 17;and a profile ignition pickup signal (PIP) from Hall effect sensor 118coupled to crankshaft 13 indicating and engine speed (N).

Continuing with FIG. 2, accelerator pedal 130 is shown communicatingwith the driver's foot 132. Accelerator pedal position (PP) is measuredby pedal position sensor 134 and sent to controller 12.

In an alternative embodiment, where an electronically controlledthrottle is not used, an air bypass valve (not shown) can be installedto allow a controlled amount of air to bypass throttle plate 62. In thisalternative embodiment, the air bypass valve (not shown) receives acontrol signal (not shown) from controller 12.

FIGS. 2-17 describe various routines carried out by controller 12. Theroutines are preferably carried out in the order in which they arenumbered, unless called by an earlier routine. However, those skilled inthe art will clearly recognize, in view of this disclosure that variousaspects of the Figures and various calculations can be rearranged innumerous orders without departing from the scope of the invention.

Referring now to FIG. 3, a routine is described for determining thedesired engine torque for use in the engine control system. First, instep 310, a driver requested wheel torque, or output shaft torque, iscalculated based on pedal position and vehicle speed. In particular, thedriver requested torque (tqo_arb_req) is calculated as a two-dimensionallookup table as a function of pedal position (PP) and vehicle speed(vspd). Next, in step 312, a limit torque (tqo_arb_lim) is determined.This limit output torque can be provided from various sources, forexample, from vehicle speed limiting, traction control limiting, or froma vehicle stability control system. When the transmission controllerprovides the limit output torque, this torque can represent maximumallowable torque that can be transmitted through the transmission. Next,in step 314, the routine calculates a driver engine torque request formanual transmissions and automatic transmissions in neutral, park, orsome driver selected gears (tqe_dd_req). Note that the tqe_dd_req is aseparate parameter then the one calculated in step 310, when tqe_arb_reqis calculated for automatic transmissions when the transmission is in agear other then neutral or park. Next, in step 316, the routine convertsdriver wheel torque request and limit torque to engine torque requestusing overall ratio G1 (which includes gear ratio, torque convertertorque ratio, transmission efficiency), and torque loss parameter LOSS,which preferably represent friction. Next, in step 318, the routineselects the maximum of the tqe_dd_req and tqe_arb_req. In this way, theroutine arbitrates the proper engine torque request taking into accountwhether an automatic transmission or manual transmission is present inthe vehicle. Further, the routine provides for automatic transmissionsoperated in a mode, such as neutral or park, when the engine is notcoupled to drive the wheels.

Referring now to FIGS. 4A and 4B, a routine is described whichcalculates a desired vehicle speed trajectory and which filters andlimits the torque request to provide various advantages as describedlater herein. First, in step 410, a routine calculates the vehicle speedtrajectory based on position of the gear selector (PRNDL), vehicle speed(vspd), and the brake pedal (BOO).

In particular, the routine calculates the maximum vehicle speed during atip-out (tq_vs_des_mx). As described later herein, this vehicle speedtrajectory is used to determine whether negative engine torque isrequired. Those skilled in the art will recognize, in view of thisdisclosure, that various other parameters can be used to provide adesired vehicle trajectory such as acceleration or deceleration.Alternatively, timers could be used to determine if a selected operatingconditions is achieved by a set time.

Continuing with FIG. 4A, the routine proceeds to step 412 where adetermination is made as to whether the pedal position is at closedpedal. This is done, for example, by checking the flag APP. Flag APP isset to minus 1 when, for example, PP is less than a predetermined valueindicated the driver has released their foot, or when the pedal angle isalmost completely released. In other words, in this implementation, theroutine determines whether the driver has positioned the pedal in themost released position, known to those skilled in the art as closedpedal. When the answer to step 412 is yes, the routine continues to step414 where the desired engine torque is rate limited. Then, in step 416,the requested torque is limited to a minimum of zero. Parametertqe_daspot represents the minimum clip on requested torque. The equationin step 414 provides a second order function, which is preferable fordrive feel. Those skilled in the art will recognize, in view of thisdisclosure, that various filtering methods could be used, such as afirst order low pass filter or a rate-limiting filter.

When the answer to step 412 is no, the routine continues to step 430 inFIG. 4 b. In other words, when the driver is not in a closed pedalcondition, which means in a part or wide-open pedal position, theroutine calculates the rate limited torque as a portion of thedifference between the current driver demand and the minimum allowedtorque (tqe_desmaf) determined in part from the misfire line asdescribed later herein. Next, in step 432, a determination is made as towhether temporary filtered torque (tqe_daspot_tmp) is greater thenfiltered desired torque (tqe_daspot). Depending on the outcome of step432, a temporary multiplier is set. In particular, this temporarymultiplier adjusts a filtering time constant for filtering enginetorque. The filter constant is set to different levels depending onwhether desired engine torque is increasing or decreasing. Step 434 setsthe multiplier for an increase in torque. Step 436, sets the multiplierfor a decrease in desired torque. Steps 438, 440, and 432 describe thedetails of how the desired engine torque is filtered. The time constant(tcdasf) is calculated in step 438. Then, the filter constant iscalculated as a function of the sample time and the parameter (tcdasf).Finally, in step 442, the filtered desired engine torque is calculatedwith a low pass filter (LPF). Those skilled in the art will recognize,in view of this disclosure that various types of filters can be usedrather than a low pass filter such as rate limiting filters, or lead lagfilters.

Referring now to FIG. 5, a routine is described which continues thedetermination of desired engine torque from FIGS. 4 a and 4 b. First, instep 510, a determination is made as to whether the requested enginetorque from step 318 (tqe_arb_req) is less than the filtered desiredengine torque (tqe_daspot). When the answer to step 510 is no, theroutine continues to step 512 when a flag (tq_dd_limit) is set to zero.Otherwise, in step 514, the desired engine torque is set equal to thefiltered engine torque. Next, in step 516, the flag (tq_dd_limit) is setto minus one. In this way, regardless of pedal angle, the filteredengine torque is applied as a minimum clip on the driver requestedengine torque.

Referring now to FIG. 6, a routine is described for determining whetherthe driver is in a closed pedal position, wherein closed pedal engineand vehicle controls are executed. First, in step 610, a flag isinitialized (tq_dd_mode=zero). This step is only executed at key-on orat part throttle conditions. This flag is used in the closed pedal statemachine to determine which state is executed. As described later herein,the state machine operates from case zero up to case 6. The flagtq_dd_mode determines which case is executed.

In step 612, a determination is made as to whether a tip out conditionis present via flag APP. In other words, a determination is made as towhether the measured accelerator pedal position is less than apredetermined value indicating the pedal is in the fully releasedposition. Those skilled in the art will recognize, in view of thisdisclosure, various ways for determining whether a closed pedal, ortip-out condition, is present. For example, vehicle speed oracceleration, engine torque determination, or various other methodscould be used.

Continuing with FIG. 6, when the answer to step 612 is no, the routinedetermines that the condition is part throttle or wide-open throttle andexecutes the routine described in FIG. 14. When the answer to step 612is yes, the routine continues to step 614, where a determination is madeas to whether the flag trg_n_t_flg is TRUE. In other words, the routinedetermines whether the engine is in the feedback engine speed controlmode. There are various places where the engine is in the closed loopengine speed control mode such as, for example, during a manual pull inwhen the transmission requests an engine speed to match the future gearratio; when the current gear does not provide engine braking asdescribed later herein; or during a neutral to drive engagement. Forexample, during a neutral to drive engagement or a manual pull in (wherethe driver changes the selected PRNDL position), the transmission candelay the actual gear change until the engine speed is brought to adesired engine speed. In these examples, the desired engine speed can beselected to equal the synchronous speed in the future gear ratio. Inthis way, transmission wear is minimized since the gear ratio can beperformed with the engine speed close to the engine speed that will beachieved after the gear change is completed. In another example relatingto when the current gear does not provide engine braking, the desiredengine torque is calculated to that the transmission input speed is at,or slightly below, the measured transmission output speed times thecurrent gear ratio of the transmission. In this way, there is no delayand transmission gear clunk is minimized when positive powertrain outputtorque is again applied. Stated another way, the desired engine speedcan be set to (or slightly below) the synchronous speed, where thesynchronous speed is based on the transmission state (selected gearratio) and the transmission output speed. Such a method can be used whenthe current selected transmission ratio does not provide engine braking.In this speed control, as described later herein, a desired torque isselected to cause the speed error to approach zero. As described, torquecontrol can be accomplished via various engine-operating parameters suchas air/fuel, ignition timing, throttle angle, or any other availabletorque actuator.

When the answer to step 614 is no, the state machine is called and thecase is executed which corresponds to the current condition of flagtq_dd_mode in step 616. Otherwise, the routine continues to 618 wherethe flag is set to 7. Then, the desired engine torque is calculatedusing a PI controller known to those skilled in the art as aproportional integral controller based on an engine speed errorcalculated from the difference between the desired engine speed (Ndesminus N).

Referring now to FIG. 7, case zero of the state machine is described.Case zero is generally called to initialize the state machine. First, instep 710, a determination is made as to whether the requested arbitratedtorque is greater than a small positive calibratable engine torque(TQE_SML_POS). When the answer to step 710 is yes, the state machineflag is set to 1 in step 712. Otherwise, the state machine flag is setto 2 in step 714.

Referring now to FIG. 8, case 1 of the state machine is described. Asdescribed above, case 1 is called when flag tqe_dd_mode is equal to 1 instep 616. In step 810, a determination is made as to whether the desiredengine torque is less than or equal to the calibratable small positivetorque (TQE_SML_POS). When the answer to step 810 is yes, the flagtqe_dd_mode is set to 2 in step 812.

Referring now to FIG. 9, case 2 of the state machine is described.First, in step 910, a determination is made as to whether the currentactual vehicle speed (vspd) is greater than the sum of the maximumallowed speed during the tip out condition (tq_vs_des_mx) plus theallowed over-speed error (vsdeltogoneg). The allowed over-speed errorcan be a single value or can vary with engine operating parameters. Forexample, depending on selected gear ratio and vehicle speed, it may bedesirable to have different thresholds of allowed over-speed error. Suchan approach may reduce excessive shifting, also known as shift busyness.When the answer to step 910 is yes, the state machine flag (tq_dd_mode)is set equal to 3. In other words, when the actual vehicle speed isgreater than the desired vehicle speed trajectory, plus the tolerancevalue, the state machine then executes in the next call of step 616,case 3, which executes a torque crossing from positive powertrain outputtorque to negative powertrain output torque, as described later hereinwith particular reference to FIG. 10. As described above, those skilledin the art will recognize, in view of this disclosure that various othervehicle parameters can be used to calculate the desired vehicle speedtrajectory and determine if the actual vehicle trajectory is below thedesired vehicle trajectory.

When the answer to step 910 is no, the routine continues to step 914,where a determination is made as to whether the torque converter islocked. When the answer to step 914 is no, the routine continues to step918. In step 918, a positive output torque is provided including closedloop control using torque converter input and/or output speeds. In thisparticular case, a desired engine speed is calculated to be greater thanthe measured torque converter output or turbine speed. This desiredengine speed is used with a closed loop proportional integral (PI)controller to calculate a desired engine torque request. In this way,feedback control is used to maintain a positive output torque. Theparameter (TQ_N_SML_POS) is a calibratable parameter to provide a safetyfactor that minimizes inadvertent zero torque crossings due to externalfactors such as road grade. In other words, the controller's objectiveis to maintain engine speed greater than torque converter output speed.Those skilled in the art will recognize in view of this disclosure thatadditional feedback can be included, wherein such feedback could be fromsensors such as a torque sensor, mass airflow sensor, or other sensorsused in torque or speed control.

Alternatively, when the torque converter is locked, the desiredarbitrated engine torque is set to the small positive torque(TQE_SML_POS). In this case, the powertrain is controlled to provide apositive output torque and minimize inadvertent transitions through thezero torque point. Since the torque converter is locked, an open loopcontrol approach is used where feedback from torque converter input andoutput speeds based on a torque converter model are not used. However,other feedback variables can be used in providing the torque controlsuch as, for example, a torque sensor or a mass airflow sensor. Inparticular, torque transmitted by the powertrain (engine output torque,transmission torque, or wheel torque) can be estimated based onoperating conditions such as, for example, mass airflow, manifoldpressure, engine speed, ignition timing, coolant temperature, and otheroperating conditions.

By providing such control of maintaining positive powertrain output,inadvertent zero torque crossings will be minimized and improved vehicledrive feel can be achieved.

Referring now to FIG. 10, case 3 of the state machine is described.First, in step 1010, a determination is made as to whether thearbitrated requested engine torque is less than a small negative outputtorque (TQE_SML_NEG), or the small negative torque is a predeterminedcalibratable parameter. When the answer to step 1010 is yes, then thestate machine flag tq_dd_mode is set to 4 in step 1012. Otherwise, instep 1014, the requested engine torque is slowly decremented to gentlypass through the zero torque point. In this way, once the negativeengine torque is provided, the routine will transition to case 4, anduntil the negative engine torque is provided, the routine will provide agradual decrease from the small positive torque to the small negativetorque so that clunk occurring at the zero torque point is minimized.

Referring now to FIG. 11, case 4 of the state machine is described.First, in step 1110, a determination is made as to whether a largenegative engine torque is required by determining if flag (rdy_very_neg)is TRUE. Typically, the flag is set TRUE by the transmission controlsystem to indicate that the torque converter is locked. In other words,various types of torque converters cannot be locked when the powertrainis transmitting large negative torques. Thus, the present invention canprevent large negative engine torques until the torque converter islocked, if such a torque converter is used. When the answer to step 1110is yes, the state machine flag (tq_dd mode) is set to 5 in step 1112.Otherwise, a determination is made as to whether the torque converter islocked in step 1114. When the torque converter is locked, the requiredengine torque is set to a small negative value (TQE_SML_NEG), which ispredetermined calibratable value. In step 1116, the negative enginetorque is provided in an open loop mode without feedback from the torqueconverter input and output speeds. Otherwise, in step 1118, closed loopengine speed control is provided where the desired engine speed iscalculated to be slightly less than the torque converter output speed.Thus, in step 1118, feedback from the torque converter input speed andoutput speed is utilized to minimize inadvertent zero torquetransitions.

Referring now to FIG. 12, case 5 of the state machine is described. Instep 1210, a determination is made as to whether the current vehiclespeed (vspd) is greater than the maximum allowed vehicle speedtrajectory value (tq_vsdes_mx). When the answer to step 1210 is yes, theroutine continues to step 1212 where state machine flag (tq_dd_mode) isset to 6.

Referring now to FIG. 13, case 6 of the state machine is described.First, in step 1310, a determination is made as to whether measuredvehicle speed (vspd) is less than equal to the desired vehicle speedtrajectory plus a predetermined calibratable value (TQ_VS_DESHYS). Whenthe answer to step 1310 is yes, the routine continues to step 1312 wherethe state machine flag (tq_dd_mode) is set to 5. Otherwise, the routinecontinues to step 1314 where feedback control vehicle speed is executedto provide the desired deceleration rate and the desired vehicle speedtrajectory. In particular, a PI controller known to those skilled in theart as a proportional integral controller is used with the desiredmaximum allowed vehicle speed and the actual speed to calculate thedesired engine torque. In this way, engine torque control is provided togive a desired vehicle trajectory.

If the state machine is called and none of the cases are executed, thedefault case is case zero.

Referring now to FIGS. 14 a and 14 b, a routine is described for ratelimiting desired engine torque when desired powertrain output isincreasing. In step 1410, a determination is made as to whether thedesired engine torque is greater than the current requested enginetorque. In other words, a determination is made as to whether thedesired engine output is increasing. When the answer to step 1410 isyes, a determination is made in step 1412 as to whether the currentengine requested torque is less than or equal to a small negative torquevalue (TQE_SML_NEG). When the answer to step 1412 is yes, the routinecontinues to step 1414, where the desired engine torque is rate limitedat a first rate determined by function G1. In other words, when thedesired engine torque is increasing but negative and less than apredetermined negative engine torque, the desired engine torqueincreasing rate is limited to a first predetermined rate, wherein thepredetermined rate is dependent on the transmission gear selected or thecurrent transmission gear ratio. When the answer to step 1412 is no, theroutine continues to step 1416, where a determination is made as towhether the current requested engine torque is less than a smallpositive calibratable value (TQE_SML_POS). In other words, adetermination is made as to whether the current requested engine torqueis near the zero torque point. When the answer to step 1416 is yes, theroutine continues to step 1418, where the desired engine torqueincreasing rate is limited based on function G2. Generally, the maximumallowed rate of increase of engine torque in this region (near the zerotorque point) is less than the allowed increasing engine torque rateoutside of this region. When the answer to step 1416 is no, the routinecontinues to step 1420, where engine torque increasing rate is limitedto a third predetermined rate based on function G3. Stated another way,that allowed increasing rate of torque is greater when for the regionsaway from the zero torque region.

In this way, the present invention provides for three different engineincreasing torque rate limits depending on the current engine torquevalue. In particular, when desired engine torque is increasing and alarge negative value, it is rate limited at a first value. When desiredengine torque is increasing near zero torque point, it is rate limitedat a second, generally lower rate. Finally, when desired engine torqueis increasing and a large positive value, it is rate limited at a thirdrate. In addition, any combination of the above three rate limits may beused. For example, engine torque may be limited only when transitioningthrough the zero torque point, or engine torque may be limited only whentransitioning through the zero torque point and when increasing abovezero torque, engine torque may be limited only when transitioningthrough the zero torque point and when increasing below zero torque.Additionally, rate limits can be set as a function of the current, orselected, gear ratio, since different rate limits may be appropriatedepending on the actual transmission gear, or based on the selected gearas indicated by the transmission lever (PRNDL). Also, as describedherein, rate limiting may be used for decreasing torque when passingthrough the zero torque region.

From step 1414, the routine continues to step 1422, where adetermination is made as to whether the current requested engine torqueis greater than the rate limited engine torque. When the answer to thisis yes, the desired engine torque is set equal to the rate limitedtorque and a rate limiting flag (tq_dd_limit) is set to 1. Otherwise,the flag is set to zero in steps 1424 and 1426. From step 1418, theroutine continues to step 1428, where the same determination as step1422 is made. When the answer to step 1428 is yes, the desired enginetorque is set equal to the rate limited engine torque and the flag(tq_dd_limit) is set to 2 is step 1430. Otherwise, in step 1432, theflat is set to zero. From step 1420, the same determination as in steps1422 and 1428 is made in step 1434. When the answer to step 1434 is yes,the desired engine torque is set to equal to the rate limited value andthe flag is set to 3 in step 1436. Otherwise, in step 1438, the flag isset to zero.

Referring now to FIG. 15, a routine is described for arbitrating betweenvarious torque limits and the desired rate limited torque request. Insteps 1510, 1512, and 1514, the rate limited desired engine torquerequest is compared with the various maximum torque limits that preventengine speed from becoming greater than a predetermined value(tqe_rpm_lim) and which prevent torque being requested which is greaterthan the maximum allowable torque transmitted through the transmission(tqe_max_tran).

Referring now to FIGS. 16 a and 16 b, a routine is described forcontrolling engine torque while maintaining a minimum airflowrequirement. In particular, the following routine provides a method toprevent engine stalls when there is a rapid decrease in required enginetorque.

First, in step 1610, anti-stall torque line (tqe_antistal) iscalculated, which is the minimum indicated torque allowed as a functionof engine speed minus desired idle speed and the torque control source(tq_source). Parameter tq_source is the dominant determinant of thetorque reduction, i.e., whether vehicle speed limiting, tractioncontrol, or shift modulation are limiting torque. Thus, since dependingon which limit is controlling, a more aggressive position can be takenon how close to the anti-stall torque line the engine is operated.

Next, in step 1612, the desired engine torque arbitrated request iscompared with the anti-stall torque line and the maximum of theseparameters is selected. Next, in step 1614, the equivalent indicatedengine torque at the minimum allowed airflow and mapped spark valuebelow which engine misfires occur is called. This value is determined asa function of engine speed. Next, in steps 1616 and 1618, the transformof engine required idle airflow is determined. First, a multiplier(idle_am_mul) is determined as a function of the difference between thedesired engine speed and the actual engine speed, and the differencebetween the current vehicle speed and a minimum vehicle speed at whichidle speed control is engaged (minmph). FIG. 16 c illustrates an exampletable showing that as the difference in vehicle speed or difference inengine speed becomes smaller, the minimum allowed airflow is graduallyadjusted to become equal to the airflow required at idle conditions.

Then, in step 1618, the multiplier is used to adjust the requiredairflow to maintain a desired engine speed at idle conditions. Then, instep 1619, this adjusted airflow is converted to a load value bydividing by the number of cylinders (numcyl_(—)0), engine speed (N), andthe amount of air that fills the cylinder at standard temperature andpressure (sarchg). Next, in step 1620, this desired load is converted toa torque using the conversion factor (TQ_(—)2_LOAD). Finally, in step1622, the maximum of the torque due to minimum airflow from misfires andthe torque due to the minimum air to guarantee engine idle speed controlis selected.

Continuing with FIG. 16 b, this selected torque is then converted to anairflow request in step 1624. Next, in step 1626, this selected torquerequest is converted from an indicated torque to an engine brake torquerequest by subtracting the torque losses (tqe_los). Finally, in step1634, the engine torque request for scheduling the required airflow forthe electronic throttle control system is selected at the maximum of theparameter determined in step 1626 and the current engine brake request.

In this way, according to the present invention, when engine and vehicleoperating conditions are away from an idle speed control range, engineairflow can be reduced below the required engine airflow for maintainingidle speed. In this way, it is possible to provide large negative enginebrake torques to maintain vehicle trajectory under a variety of vehicleoperating conditions. However, as the vehicle conditions approach anengine idle speed region, airflow is increased to a required engine idlespeed control level. In this way, even despite the engine airflow delaysdue to manifold volume, it is possible to maintain robust idle speedcontrol as well as provide large negative engine braking ability.

Referring now to FIG. 17, a routine is described for calculating adesired vehicle trajectory, which is called in step 410 of FIG. 4 a.First, a determination is made as to whether flag (APP) is less thanzero. In other words, a determination is made in step 1710 as to whethera closed pedal (tip-out condition) is present. When the answer to step17 is yes, the desired closed pedal acceleration (ct_accl_des) iscalculated as a function of the current vehicle speed and the gearselected position (PRNDL). Next, in step 1714, a determination is madeas to whether the brake pedal is released. When the answer to step 1714is yes, a determination is made as to whether the brake pedal engagementduration (boo_duration) is greater than zero in step 1716 indicating thefirst pass through the routine since the brake was depressed. When theanswer to step 1716 is yes, the vehicle speed release value(vs_on_release) is set equal to the current vehicle speed and the lastbrake engagement duration is set equal to the current brake engagementduration value in step 1718. Next, in step 1720, a determination is madeas to whether the first brake engagement duration (boo_(—)1st) isgreater than a predetermined duration (tq_boo_long) and the flag(tq_frz_vsboo) is true. Flag (tq_frz_vsboo) is a selection flag thatallows using brake duration in determing the maximum allowed vehiclespeed trajectory.

Parameter (tq_boo_long) represents the braking duration after which themaximum allowed vehicle speed trajectory will be held constant. In otherwords, if the driver simply taps the brake, the maximum allowed vehiclespeed will continue to ramp toward zero after the brake is released.However, if the driver holds the brake pedal for longer than apredetermined value, the maximum allowed vehicle speed is held to thevehicle speed when the brake was released. This can give the driver theability to set a desired speed using the brake on a long downhill grade.

Continuing with FIG. 17, when the answer to step 1720 is yes, themaximum allowed vehicle speed is set to parameter vs_on_release in step1722. When the answer to step 1720 is no, the maximum allowed vehiclespeed is set to the previously set maximum allowed vehicle speed plus adesired acceleration times the sample time in step 1724. Step 1724represents where maximum allowed vehicle speed is gradually rampedtoward zero.

When the answer to step 1710 is no, the brake engagement duration andthe first brake engagement duration are both set to zero and the desiredmaximum vehicle speed is set to the current vehicle speed in step 1720.When the answer to step 1714 is no, the maximum desired vehicle speed isset to the current vehicle speed and the brake engagement duration isincremented by sample time in step 1722.

In this way, the desired vehicle trajectory is determined based on thecurrent vehicle speed and the position of the gear selector (PRNDL).Further, the desired vehicle trajectory is adjusted based on actuationor engagement of the brake pedal. In particular, the length ofengagement of the brake pedal is used to adjust the desired vehicletrajectory. For example, the desired vehicle speed trajectory isdecreased while the brake pedal is engaged and set to the value of theactual vehicle speed when the brake pedal is released in some cases. Inthis way, improved drive performance can be achieved since allparameters indicative of the driver's desired vehicle operation arebeing incorporated.

Referring now to FIG. 17B, an example of operation is described whilethe accelerator pedal is released (i.e., closed pedal operation). Thetop graph shows the brake actuation signal and the bottom graph showsthe maximum allowed vehicle speed trajectory. At time t1, the brake isdepressed and released at time t2. While the brake is pressed themaximum allowed vehicle speed is set to the current vehicle speed, andthus no control action is taken. Since time difference □t1 is less thanthe predetermined brake duration, the ramping of the maximum allowedvehicle speed is then continued until the brake is depressed again attime t3. The brake is then released at time t4. Since time difference□t1 is greater than the predetermined brake duration, the vehicle speedupon release at time t4 is captured and held as the maximum allowedvehicle speed.

Referring now to FIG. 18, a routine is described for determining, insome cases, whether the torque converter should be locked. Inparticular, the routine determines whether the torque converter can belocked during a closed pedal operation. First, in step 1810, adetermination is made as to whether the state machine is in case 3 andwhether the torque converter is presently unlocked. When the answer tostep 1810 is yes, the torque converter can be locked in step 1820. Inother words, the torque converter can be locked when the engine torqueis less than a small, predetermined negative torque value. In otherwords, the torque converter can be locked after the vehicle hastransitioned through the zero torque point. This is especiallyadvantageous if it is desired to unlock the torque converter when thedriver again depresses the accelerator pedal and requests positivepowertrain output. In particular, under this situation, the torqueconverter can be unlocked and thus provide a rapid amount of powertrainoutput, thus improving vehicle performance feel.

Referring now to FIG. 19, a graph illustrating typical operationaccording to the present invention is shown. The graph plots enginebrake torque versus time for a tip-out. The dash line illustrates thedesired engine torque value determined from, for example, the driveractuated element. The solid line indicates the actual engine torqueproduced. At time T1, the driver releases the foot pedal and the tip-outsituation is begun. The algorithms, according to the present inventionas described herein, first reduce the engine torque by a predeterminedamount. Then, the engine torque is gradually decreased at apredetermined rate, which is determined by a selected tip-out torquedecrease trajectory. The engine torque is decreased until it reaches asmall positive value (TQE_SML_POS). Maintaining the torque converterinput speed greater than the torque converter output speed holds thissmall positive torque. Then, at time T2, there is a decision to providenegative engine torque based on the vehicle trajectory. In particular,at time T2, the actual vehicle speed becomes greater than the maximumallowed vehicle speed plus a predetermined calibratable value. Startingat time T2, the engine torque is gradually decreased at a predeterminedrate through the zero torque point. Also, in this region, torque linecan be used using the torque converter input and output speeds to learnthe zero torque point and to update the engine torque model. Then, attime T3, a small negative torque is held by maintaining the torqueconverter output speed greater than the torque converter input speed.This small negative torque is held for a short period until, at time T4,a decision is made to lock the torque converter to provide high levelsof negative torque. At time T4, the torque converter is locked. Then,the negative torque level is selected to maintain the desired vehiclespeed trajectory. The negative torque level is selected such that theactual vehicle speed is generally below the maximum allowed vehiclespeed.

Referring now to FIGS. 20 and 21, two circuit torque converter 11 a isshown. FIG. 20 shows the two circuit torque converter clutch disengaged,while FIG. 21 shows the two circuit torque converter clutch engaged. Twocircuit torque converter 11 a is shown having input shaft 13 a, which iscoupled to engine crankshaft 13, and output shaft 17 a, which is coupledto transmission input shaft 17. Two circuit torque converter 11 a hasconverter clutch 200 a. Two circuit torque converter 11 a is disengagedby supplying pressure to the clutch control side of the clutch. Thepressure is exhausted through the impeller side of the converter. Theexhaust fluid is sent to a cooler. In particular, the clutch controlpressure must work against the pumping action of the impeller. To applythe converter clutch, fluid flow is reversed.

Referring now to FIGS. 22 and 23, three circuit torque converter 11 b isshown. FIG. 22 shows the three circuit torque converter clutchdisengaged, while FIG. 23 shows the three circuit torque converterclutch engaged. Three circuit torque converter 11 b is shows havinginput shaft 13 b, which is coupled to engine crankshaft 13, and outputshaft 17 b, which is coupled to transmission input shaft 17. Two circuittorque converter 11 b has converter clutch 200 b. In FIG. 22, fluid issupplied to both the impeller side and to the converter clutch controlcircuit of the converter; this prevents the clutch from being engaged.The purpose of orifice 202 b on the converter inlet side is to reducethe amount of pressure on the converter side of the clutch. Thehydraulic pressure in the front chamber becomes greater than pressure inthe rear chamber, holding the converter clutch away from the convertercover and releasing lockup. During lock-up mode, in FIG. 23, fluid isallowed to exhaust through the clutch control circuit, thereby allowingthe converter clutch piston to apply. Hydraulic pressure in theconverter side of the clutch causes the converter clutch to presstightly against the converter cover. Lock-up occurs, and power istransmitted directly to transmission 15 with no fluid slippage.Converter in oil is fed directly, without an orifice. Converter outputis restricted by orifice 204 b to ensure the pressure builds up on theconverter side of the lockup clutch.

The inventors of the present invention have found that torque converter11 a is more difficult to lock when transmitting large negative torque(impeller spinning much slower than turbine) than torque converter 11 b.A potential explanation of this is that when the impeller is spinningslower than the turbine, the turbine is pushing oil into the impeller,rather than the other way. It is then hard to build pressure on theturbine side to push the clutch on.

However, those skilled in the art will recognize, in view of thisdisclosure, that the method of the present invention is not limited totwo circuit torque converters. In particular, this aspect of the presentinvention is applicable to any torque converter that would be difficultto lock when transmitting large negative torque values. For example,this difficulty may be due to inability to build hydraulic pressure orinability to exhaust hydraulic pressure. Typically, this type of torqueconverter has insufficient hydraulic pressure to be locked whentransmitting a predetermined amount of negative torque. Thispredetermined amount of negative torque can be determined using torqueconverter input and output speeds. For example, when output speed isgreater than input speed by a predetermined amount, the torque converterhas insufficient hydraulic pressure to be locked.

Further, the inventors have recognized that it is possible to minimize“clunk” by providing an un-locked torque converter when passing throughthe zero torque point (or transmission lash zone). And, at the sametime, provide maximum availability of negative powertrain torque with alocked torque converter by locking the torque converter aftertransitioning through the lash zone.

1-4. (canceled)
 5. A method for controlling a powertrain coupled to avehicle, the powertrain having an internal combustion engine coupled toa transmission, the vehicle operated by a driver, the method comprising:determining a desired vehicle trajectory; adjusting an engine operatingparameter to maintain positive powertrain output when an actual vehicletrajectory is below said desired vehicle trajectory; and adjusting saidengine operating parameter to transition from positive powertrain outputto negative powertrain output when said actual vehicle trajectory isabove said desired vehicle trajectory. The method recited in claim 1wherein said vehicle trajectory is a vehicle acceleration trajectory.6-12. (canceled)
 13. A method for controlling a powertrain coupled to avehicle, the powertrain having an internal combustion engine coupled toa transmission via a torque converter having an input speed and anoutput speed, the vehicle operated by a driver, the method comprising:indicating when the driver requests a decrease in powertrain output; inresponse to said indication, determining a desired vehicle trajectory;and maintaining positive powertrain output when an actual vehicletrajectory is below said desired vehicle trajectory.
 14. The methodrecited in claim 13 wherein said maintaining further comprisesmaintaining powertrain output by adjusting an engine operating parameterto sustain the torque converter input speed greater than the torqueconverter output speed.
 15. The method recited in claim 13, wherein saiddesired vehicle trajectory is based on vehicle operating conditions. 16.The method recited in claim 15 wherein said vehicle-operating conditionsare selected from the group consisting of vehicle speed, actual gearratio, and selected transmission gear.
 17. A method for controlling apowertrain coupled to a vehicle, the powertrain having an internalcombustion engine coupled to a transmission via a torque converterhaving an input speed and an output speed, the vehicle operated by adriver, the method comprising: indicating when the driver requests adecrease in powertrain output; in response to said indication,determining a desired vehicle trajectory; and maintaining positivepowertrain output when an actual vehicle trajectory is below saiddesired vehicle trajectory by adjusting an engine operating parameter tosustain the torque converter input speed greater than the torqueconverter output speed. 18-19. (canceled)